Continuous sol-gel process for producing silicate-containing glasses or glass ceramics

ABSTRACT

A continuous sol-gel process for producing silicate-containing glasses and glass ceramics is proposed, comprising the following steps:
     (a) continuously feeding a silicon tetraalkoxide, a silicon alkoxide with at least one non-alcoholic functional group and an alcohol into a first reactor (R1), and at least partially hydrolyzing by the addition of a mineral acid to obtain a first product stream (A);   (b) continuously providing a second product stream (B) in a second reactor (R2) by feeding a metal alkoxide component or continuously mixing an alcohol and a metal alkoxide component;   (c) continuously mixing product streams (A) and (B) in a third reactor (R3) for producing a presol to obtain a third product stream (C);   (d) continuously adding water or a diluted acid to the product stream (C) to obtain a sol (gelation);   (e) continuously filling the emerging sol into molds to obtain an aquagel;   (f) drying the aquagels to obtain xerogels;   (g) sintering the xerogels to obtain silicate-containing glasses and glass ceramics.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. Application No.15/673,733, filed on Aug. 10, 2017, which, in turn, claimed the benefitof priority to the EPO Application EP 16184064.0, filed on Aug. 12,2016. The content of these prior Applications is incorporated byreference herein in their entirety.

FIELD OF THE INVENTION

The invention pertains to the field of inorganic chemistry and relatesto a continuous process for producing silicate-containing glasses orglass ceramics.

BACKGROUND ART

Three-dimensional quartz glass bodies can be prepared by the so-calledsol-gel process. The principle of this process is based on an acid- orbase-catalyzed hydrolysis followed by gelation because of condensationreactions. The originally liquid sol undergoes a transition through astable liquid dispersion of oxide particles into a gel-like and finallysolid state. The thus obtained aquagel is subsequently dried to axerogel, and sintered to quartz glass. The final product is glassy. Theporosity and morphology of the products can be adjusted by the additionof different additives, or through the drying schedule. In contrast toconventional quartz glass production by melting the raw materials atvery high temperatures, the shaping takes place at room temperature inthe sol-gel process. The glass bodies prepared with this technologyusually need not be reworked, which is both more time-efficient and lessexpensive.

The starting materials of a sol-gel synthesis are low molecular weightmetallic alkoxide compounds. The first step of this synthesis is thehydrolysis of the alkoxides in the presence of an acid or base. As aresult of this process, unstable hydroxy compounds (a) are formed, whichsometimes may easily oligomerize. The solution formed is a sol. In acondensation reaction, individual compounds grow together by theformation of siloxane bridges (Si—O—Si) (b). This process continuesuntil all monomers are consumed. A contiguous network is not yet formed.Under suitable reaction conditions, all generated particles are in auniform size distribution of a few nanometers. The reaction rates of thehydrolysis and condensation can be influenced through the medium, pH andconcentration, and proceed simultaneously (c). The process was describedin some detail by Nogami et al. in Journal of Non-crystalline Solids,37, pp. 191-201 (1980).

In a suitable environment, a sol remains stable for several weeks, inpart even months. The gelation takes place by condensation to formsiloxane linkages. Now, the eponymous step of the synthesis, the sol-geltransition, is reached. From the loose particles of the sol, athree-dimensional network has formed, which is soaked with the solvent.The sol has become a gel.

After the gelation is complete, the aquagel is dried to a xerogel. Thecomplete evaporation of the solvent causes a stronger cross-linking ofthe entire network. This step results in a compact, highly cross-linkedand resilient material:

-   M = metal or metalloid, R = residue,-   n = 1-4-   a = 0-3-   b = 0-3-   c = 0-3

In the last step, the xerogel is sintered to quartz glass.

From the prior art, a large number of processes are known that deal withquartz glass production in general and the sol-gel process inparticular.

From European Patent EP 0 131 057 B1 (Seiko), a batch process forproducing quartz glass is known in which a hydrolyzed solution of ametal alkoxide of the formula M(OR)_(x) is provided at first, from whicha sol (colloidal solution) is formed. After gelation, the sol is driedto a xerogel. Subsequently, the xerogel is sintered to quartz glass.

According to European Patent EP 0 807 610 B1 (Lucent), a process isdisclosed for producing a silica sol, which essentially consists ofnon-agglomerated silica, in which a starting mixture of silica particlesin water is prepared, and the silica sol is formed from the mixture byshear-mixing. An alkaline substance without a metal cation is added tothe sol to adjust the pH to from 6 to 9.

European Patent EP 1 251 106 B1 (Fitel) claims a process in which a solis provided by mixing silica particles and water, wherein said particleshave a surface area of from 5 to 25 m²/g, contain at least 85% sphericalparticles, and the weight ratio of the particles to water is larger than65%. Subsequently, the pH is adjusted to from 10 to 13 using a base, anda gelling agent is added to the sol. Tetramethylammonium hydroxide andtetraethylammonium hydroxide are used as bases.

From European Patent Application EP 1 258 457 A1 (Degussa), a process isknown in which a silicon alkoxide is hydrolyzed, followed by theaddition of Aerosil® Ox50, which is employed because of its specialproperties, its particle size and its surface area.

European Patent EP 1 320 515 B1 (Degussa) relates to a process in whichtwo solutions are prepared, and then combined for reaction. Solution Ais an aqueous acidic dispersion (pH 1.5) of a fumed silica (e.g.,Aerosil® Ox50), while solution B is an aqueous basic dispersion (pH10.5-13) of a fumed silica (e.g., Aerosil® Ox200). The molar ratio ofH₂O to SiO₂, the molar ratio of the Si compound in solution A to that insolution B, and the resulting pH of the mixture C (after the twosolutions have been combined) are the critical features for obtainingthree-dimensional bodies that are larger than 2 cm.

In European Patent Application EP 1 606 222 A1 (Degussa), a process isclaimed in which either a sol or a corresponding precursor is preparedfrom a silicon alkoxide. Subsequently, the sol is hydrolyzed, and thencolloidal SiO₂ is added.

According to European Patent Application EP 1 661 866 A1 (Evonik), anaqueous dispersion of fumed silica (colloidal silica) is provided, itspH is adjusted to from 2 to 0.5, and then TEOS is added. The thusobtained sol is subsequently adjusted to basic and cast into a mold,where it solidifies to a gel.

In European Patent Application EP 1 700 830 A1 (Degussa), a process isproposed in which an aqueous dispersion of pyrogenic metal oxide isprovided at first, and a metal oxide, which was previously hydrolyzed bythe addition of water, is added thereto. The thus obtained sol issubsequently cast into a mold, in which it gels.

European Patent Application EP 1 770 063 A1 (Dynax) relates to a processfor preparing silica aerogels with a defined pore diameter and porediameter distribution, in which silicon components containing bothhydrolyzable and hydrophobic functional groups, preferablymethyltrimethoxysilane, are hydrolyzed in an acidic aqueous surfactantsolution. Non-ionic (e.g., polyoxyethylene alkyl ether, polyoxypropylenealkyl ether), cationic (cetyltrimethylammonium bromide or chloride) oranionic (sodium dodecylsulfate) surfactants are employed as possiblesolvents.

The process of European Patent Application EP 2 064 159 A1 (Degussa)includes the following steps: adding fumed SiO₂ to the acidic aqueousmedium, and subsequently adding a silicon alkoxide to the dispersionobtained. The molar ratio of silicon dioxide to silicon alkoxide is tobe from 2.5 to 5. This is a batch process, in which fumed silica isprovided first, and then the silicon alkoxide is added.

In European Patent Application EP 2 088 128 A1 (Degussa), a process isproposed in which fumed SiO₂ is added to water adjusted to acidic, and asilicon tetraalkoxide is added to the thus obtained dispersion. The pHis adjusted again, and the mixture is placed into a container, where thesol solidifies to a gel. Thereafter, it is dried to a xerogel andsintered to a glass product.

From the international patent application WO 2013/061104 A2 (DebreceniEgyetem), a continuous process for preparing alcogels, aerogels andxerogels is known in which silanes are hydrolyzed in the presence ofbasic catalysts and a specific aqueous-organic solvent system and agelation retarder, and inert particles are introduced into the solution.

A disadvantage of the batch processes of the prior art is the fact thatdefined discrete amounts can be prepared at a time, which may result inquality differences. The batch production favors the inclusion of airbubbles in the glass, which also leads to quality reduction. A furtherdisadvantage is the extensive cleaning of all systems required aftereach run. In addition, a continuous process offers simpler possibilitiesof scaling up.

It has been the object of the present invention to overcome the abovedescribed drawbacks and, in addition, to produce multicomponent glassesand glass ceramics with a high refractive index. One possibility is toperform the synthesis in a continuous process and to feed alkoxidecomponents, which lead to an increase of the refractive index. Becauseof the continuous reaction mode, any amounts of silicate-containingglasses and glass ceramics of a constant high quality can be prepared.

BRIEF DESCRIPTION OF THE DRAWINGS

The single the FIGURE shows transmission data of inventive products incomparison to conventional silica glass.

DESCRIPTION OF THE INVENTION

The present invention relates to a continuous sol-gel process forproducing silicate-containing glasses and glass ceramics, comprising thefollowing steps:

-   (a) continuously feeding a silicon tetraalkoxide, a silicon alkoxide    with at least one non-alcoholic functional group and an alcohol into    a first reactor (R1), and at least partially hydrolyzing by the    addition of a mineral acid to obtain a first product stream (A);-   (b) continuously providing a second product stream (B) in a second    reactor (R2) by feeding a metal alkoxide component or continuously    mixing an alcohol and a metal alkoxide component;-   (c) continuously mixing product streams (A) and (B) in a third    reactor (R3) for producing a presol to obtain a third product stream    (C);-   (d) continuously adding water or a diluted acid to the product    stream (C) to obtain a sol (gelation);-   (e) continuously filling the emerging sol into molds to obtain an    aquagel;-   (f) drying the aquagels to obtain xerogels;-   (g) sintering the xerogels to obtain silicate-containing glasses and    glass ceramics.

The term “presol” as mentioned in step (c) of the process according tothe invention refers to dispersion/solution of product stream (C) beforewater or acid or, in case of silica glass ammonia, is added. In contrastthereto, the term “sol” as used in step (d) of the inventive processrefers to the mixture of presol and water/acid/ammonia and ischaracterized by complete hydrolysis of the remaining alcohol moieties.The OH-groups formed at the silicon further react with each other underelimination of water, a process referred to as gelation.

Surprisingly, it has been found that the novel continuous process solvesall of the above described varied problems simultaneously and entirely.Not only does the process allow for the production of arbitrary andtherefore always different amounts of product, but also the synthesisleads to products of a constantly high quality.

It was further surprisingly found that the continuous process accordingto the invention allows the incorporation of rather high amounts ofmetal alkoxide component. The presence of a metal oxide in the solusually speeds up the gelation process, which leads to problems duringthe process as the viscosity of the product stream in the systemincreases, reducing its flowability in the process. This can lead to theproduct stream becoming solid, thereby halting the process. It wassurprisingly found that the undesired decrease in gelation time can beavoided by conducting the process of production in a continuous manner.Also, the product obtained by the inventive process can be furtherprocessed immediately, without the need of any additional melting steps.The process according to the invention thus allows the production ofcrack-free and transparent glass or glass ceramics with a high amount ofmetal alkoxide.

In addition, due to the continuous manner in which it is conducted, theinventive process allows for a great deal of flexibility with respect tothe composition of the sol as components and amounts can be changed atany given time during the production. In turn, the new found flexibilityallows the control of the density and refractive index of the productover a wide area. The inventive process operates as a closed systemthereby eliminating the need for working in a protective atmosphere andlimiting the exposure of the worker to the chemicals used.

Further, a preferred embodiment of the process according to theinvention is to feed the starting materials of the synthesis in adegassed state, because it has been found that, without this step, gasesdissolved in the starting materials are released by the mixing becauseof an altered solubility, and may induce undesirable bubble formation.In principle, the degassing can be effected in any of the process steps(a) to (e), i.e., on the level of the starting materials, the presol, adispersion, or the sol itself. Preferably, the starting materials arealready degassed and employed in the synthesis in this form. For thesake of security, both the starting materials and the presol, thedispersion or sol may be degassed.

According to the invention, the degassing is preferably performed withultrasound. Alternatively, possible methods include:

-   vacuum degassing;-   distillation;-   reduced pressure/freezing cycles;-   thermal degassing;-   chemical methods, such as oxygen removal by chemical binding;-   gas removal by means of inert gas;-   addition of deaeration additives; and-   centrifuging;

or a combination of two or more of these measures.

In addition, the starting materials may optionally be employed in aparticle free form by using suction filters, and each mold can be filledwith freshly prepared sol. Thus, by avoiding rejects that do not complywith the specifications, mainly the profitability of the process issignificantly enhanced, all the more so since long cleaning times can bedropped, especially since the reactors that are preferably to beemployed can be easily cleaned with rinsing media.

Therefore, the present invention relates to silicate-containing glassesor silicate-containing glass ceramics according to the claims, andprocesses for producing them. Process steps (a) to (g) are furtherexplained in the following.

Process Step A

Silicon alkoxides, which are possible starting materials for theproduction of the silicate-containing glasses or silicate-containingglass ceramics according to the invention, preferably comply withformula (I)

in which R represents an alkyl radical with 1 to 6 carbon atoms. Typicalexamples include tetrapropyl orthosilicate and tetrabutyl orthosilicate,but tetramethyl orthosilicate (TMOS) and especially tetraethylorthosilicate (TEOS) are preferably employed. In a preferred embodiment,since TEOS is insoluble in water, alcoholic, especially ethanolic,solutions can be employed, in which the alcohol adopts the function of aphase-transfer agent. The silicon alkoxides may also include othersilicon compounds as additives, such as methyltriethylsilane,dimethyldiethylsilane, trimethylethylsilane, methyltriethoxysilane(MTES), triethoxyoctylsilane, octylme-thyldichlorosilane,triethoxyvinylsilane, vinyltrimethoxysilane, and the like.

In an especially preferred embodiment, the additive ismethyltriethoxysilane (MTES) and the silicon tetraalkoxide is tetraethylorthosilicate (TEOS).

It was surprisingly found that the addition of a silicon alkoxide withat least one non-alcoholic functional group leads to an increase in thetransparency of the obtained glass or glass ceramic. Without being boundby theory, it is believed that the presence of the silicon alkoxide withat least one non-alcoholic functional group prevents completecross-linking, allowing the formation of a sufficient number of openpores. Due to the open pores, any volatile material trapped in the geland glass or the glass ceramic can easily be evaporated during dryingand sintering without the danger of crack-forming. In conventionalprocess, the glass or glass ceramic might suffer from crack-formationduring drying and sintering due to volatile material still present inthe product which will lead to destruction of the glass or glass ceramicdue to build-up pressure upon heating. This problem is overcome by theinventive process. Thus, the inventive process allows the formation ofhighly transparent glass or glass ceramic which are crack-free and canbe densely sintered.

In a preferred embodiment of the inventive process, the weight ratio ofsilicon tetraalkoxide and the silicon alkoxide with at least onenon-alcoholic functional group is in the range of 30:1 to 1:5,preferably 25:1 to 1:5, more preferably 20:1 to 1:1, and in particular15:1 to 5:1. It was surprisingly found that the yield of the process andthe quality of the product, in particular with respect to transparency,could be further increased when the ratio of said two components waswithin the claimed range.

The acidic hydrolysis of the silicon alkoxides is effected in reactor R1in the presence of mineral acids, such as sulfuric acid, nitric acid andhydrochloric acid, or in the presence of acetic acid. The mentionedacids are preferably employed in an aqueous diluted form, and dilutednitric acid with a concentration of about 1.00 mol/l has provenparticularly favorable. Alternatively, aqueous hydrochloric acid, towhich surface-active substances may optionally be added, is alsosuitable. The amount of water added with the acid is addedstoichiometrically here, so that in TEOS, for example, preferably onlyone ethanolate group at a time is cleaved off and replaced by OH . Thepreferred volume ratio of alkoxide to mineral acid is from 1:1 to 20:1,more preferably from 5:1 to 15:1, and even more preferably from 7:1 to12:1. The hydrolysis is performed at a suitable temperature by feedingthe two starting materials using pumps, combining them and allowing themto react in a temperature-controlled flow reactor. When the startingmaterials are not miscible with one another, a slug flow may form in theflow reactor. Preferably, an alcohol, preferably ethanol, is added as aphase-transition agent. The temperature range of the hydrolysis is from1 to 100° C., the preferred temperature being from 10 to 50° C.Preferably, the hydrolysis is effected at 18 to 40° C., especially at 20to 25° C.

Process Step B

Metal alkoxide components, which are possible starting materials for theproduction of the silicate-containing glasses or silicate-containingglass ceramics according to the invention, are preferably organictransition metal alkoxides, preferably those of group 4-8 transitionmetals. The metal alkoxide components preferably comply with the formulaM(OR)_(n)(OH)_(m), with M = metal and R = radical (n + m corresponds tothe valence of this cation). Preferably, M is selected from the elementsof group 4 of the periodic table, Ti and Zr being preferred, while R ispreferably an organic alkyl radical with 1 to 5 carbon atoms, preferablyselected from the group of ethyl, propyl, isopropyl, butyl, isobutyl,pentyl, isopentyl or neopentyl radicals. Suitable metal alkoxidecomponents are preferably alkyl orthotitanate, in particulartetraisopropyl orthotitanate (Ti[OCH(CH₃)₂]₄) or zirconium(IV) alkoxide,in particular zirconium(IV) butoxide (Zr(OCH₂CH₂CH₂CH₃)₄).

In the second process step, the metal alkoxide component is mixed withalcohol in a reactor R2. The reactor may also be temperature-controlled.Alternatively, the neat metal alkoxide component may also be employed.The preferred volume ratio of alcohol to metal alkoxide component ispreferably from 1:5 to 5:1, more preferably from 2:1 to 1:2. Thetemperature is kept at 0 to about 40° C., preferably at 20 to 30° C.

Process Step C

While a first continuous stream of a hydrolyzed silicon alkoxidecompound was prepared in the first process step and, also continuously,a second stream of an alcoholic solution of a second metal alkoxidecomponent was produced in the second step, the mixing of the two streamsand the forming of the presol are effected in the third step. Thus,product streams (A) and (B) are combined upstream of reactor R3 by meansof a suitable mixing system. The volume ratios of the two streams (A)and (B) can be adjusted variably. The product properties of the finishedglass can be influenced thereby. A preferred volume mixing ratio of (A)to (B) is from about 10:1 to about 1:10, more preferably from about 8:1to about 1:5, even more preferably from about 4:1 to about 1:2.Depending on the quality demanded from the glass, the presol can bedegassed here by suitable degassing methods, for example, by ultrasound.The combination of product streams (A) and (B) is performed attemperatures of from 0 to about 100° C., preferably from 0 to 40° C.,more preferably at 20 to 30° C.

Process Step D

The subsequent gelation of the presol is induced by the addition ofwater or an acid when the pH is simultaneously changed. For thispurpose, water or an acid, for example, a diluted mineral acid, such assulfuric acid, nitric acid and hydrochloric acid, or else an organicacid, such as acetic acid (about 1.00 mol/l), is continuously fed to thecontinuously produced presol (product stream (C)). In process step (D),a corresponding molar quantity of water is added, so that all remainingalcoholate radicals are hydrolyzed, and the newly formed hydroxy groupssubsequently condensate. Meanwhile, the system becomes cross-linked(gelation). The mixture is preferably cooled down to 5 to 0° C. in orderthat the gelation time is not too short.

In the present process step, various additions can be made to determinethe product properties, such as the addition of cations, preferably thecations of the elements Na, K, Cs, Sr, Ba, B, Al, Zn, Y, La, Ce, Sm, Eu,Tb and Tm, or when colored glasses are desired, the cations of theelements V, Cr, Mn, Fe, Ru, Co, Ni, Cu, Au, Cd, Pr, Nd and Er, andmixtures thereof.

Reactors

According to the invention, the reaction is performed in a flow reactor,optionally with an upstream mixing element.

In the simplest embodiment, the reactors are flexible tubes made of someresistant material, such as teflon, polyamide, metal, polyethylene orpolypropylene, which may have a length of about 1 cm to about 1000 m,preferably about 5 cm to about 500 m, and more preferably from 70 cm to400 m, and on average have a cross-sectional width of about 1 to about10 mm, preferably from about 1 to about 5 mm. These flexible tubes maybe wound spirally, which significantly reduces the space requirements.For a given flow rate, the long paths correspond to the respectivelyoptimum reaction time. Such assemblies are exceptionally flexible,because the lengths of flexible tubes can be arbitrarily elongated orshortened, and can be cleaned with low expenditure. Such a reaction modecan contribute substantially to the profitability of the process.

Gelation

The presol is continuously fed from the reactor R3 and cast into molds,in which gelation can take place. Because the thus obtained aquagelsshrink in the mold during the ageing, they must be able to slide easilyin the container. For this reason, especially containers made of ahydrophobic material, such as polyethylene, polypropylene, teflon, PVCor polystyrene, are suitable here.

The aquagels obtained must be demolded for processing, or be dried inthe molds to xerogels. The demolding can be effected under particularconditions, for example, with an alcoholic solution. The dryingconditions are influenced by the vapor pressures of the solvents in thegel, i.e., the alcohols, the water, and the acids. Compliance with a lowevaporation rate keeps the gel from cracking. Conversely, long dryingtimes make the process expensive, so that a compromise must be foundhere. The drying of the aquagel to xerogel is preferably effected at atemperature gradient from room temperature to 150° C. To control thedrying atmosphere, the evaporation rate must be adjusted bycorrespondingly dimensioned openings and the optional presence of adrying solution.

Sintering

The sintering can be performed in a per se known manner. During thesintering, the remaining solvents still contained in the xerogels areremoved, and the pores in the system are closed. The sinteringtemperature is up to 1400° C., and for most products, the sintering canbe performed under normal atmosphere. Preferably according to theinvention, the sintering is performed as follows:

-   1) removing the remaining solvents;-   2) removing any undesirable organic compounds contained;-   3) closing the existing pores to form silicate-containing glasses or    glass ceramics.

In order to remove the solvents (1) and undesirable organic compoundsformed by the decomposition of carbon-containing startingmaterials/products (2), calcination is performed at temperatures withina range of from about 600 to about 1100° C., preferably about 700 toabout 900° C., more preferably about 750 to about 850° C. In step 3, theclosing of the pores is effected by sintering at temperatures from about800 to about 1400° C., preferably about 850 to about 1200° C., morepreferably at about 950 to about 1100° C.

A further object of the present invention is a silicate-containing glassor silicate-containing glass ceramic obtainable by the inventiveprocess. As mentioned above, the process according to the inventionallows the production of highly transparent and crack-free glass orglass ceramic comprising a high amount of metal oxide compound. Theinventive silicate-containing glass or silicate-containing glass ceramicis therefore characterized by a high transparency. Thesilicate-containing glass or silicate-containing glass ceramicpreferably has a transparency close or equal to that of silica glass. Inparticular, the silicate-containing glass or silicate-containing glassceramic is preferably transparent for light with a wave length in thevisible spectrum, in particular light with a wave length ranging from300 to 900 nm, preferably 350 to 850 nm and especially 380 to 780 nm.

In a preferred embodiment,the silicate-containing glass orsilicate-containing glass ceramic according to the invention has atransmission in the range of visible light of at least 70%, preferablyof at least 80% of the transmission of silica glass in the range ofvisible light, the transmission being determined by I/I₀ with I₀ beingthe initial intensity of the light. In a further preferred embodiment,the value of transmission of the silicate-containing glass orsilicate-containing glass ceramic according to the invention does notdiffer more than 30%, preferably no more than 20%, from the value oftransmission of silica glass in the range of visible light. In anespecially preferred embodiment, the silicate-containing glass orsilicate-containing glass ceramic according to the invention has atransmission comparable to the transmission of silica glass in the rangeof visible light.

In a preferred embodiment, the silicate-containing glass orsilicate-containing glass ceramic according to the invention comprisesthe metal oxide component in an amount ranging from 10 wt.-% to 60wt.-%, preferably 20 wt.-% to 55 wt.-%. In an alternative embodiment,the content of metal oxide component in the silicate-containing glass orthe silicate-containing glass ceramic is preferably 10 to 35 wt.-%,especially 10 to 25 wt.-%.

The inventive silicate-containing glass or silicate-containing glassceramic is further characterized by a high refractive index which can becontrolled via addition of metal oxide components. Here, the inventiveprocess allows the addition of rather high amounts of metal oxidecomponent, making it possible to obtain silicate-containing glass orsilicate-containing glass ceramic with high refractive indices. In apreferred embodiment, the silicate-containing glass orsilicate-containing glass ceramic has a refractive index nD of 1.45 to1.8, preferably 1.48 to 1.75, in particular 1.5 to 1.7.

In a preferred embodiment, the silicate-containing glass orsilicate-containing glass ceramic according to the invention has atransmission which differs no more than 30%, preferably no more than 20%from the transmission of silica glass in the range of visible light, anda refractive index nD of 1.45 to 1.8, preferably 1.48 to 1.75, inparticular 1.5 to 1.7.

EXAMPLES

The term “room temperature” as used in the present invention, refers toa temperature of 20° C.

Example 1 SiO_(2/)TiO₂ Glass

Ethanol was supplied by a first pump, HNO₃ (1.0 mol/l) by a second one,and TEOS and MTES by two other pumps. The liquids were combined inflexible tubes by means of T pieces. The mixture had the followingcomposition:

-   TEOS 45.0% by volume-   MTES 4.5% by volume-   EtOH 45.0% by volume-   HNO₃ 5.5% by volume

At room temperature, the mixture entered a first PA flexible tube(reactor R1) having a length of 200 m and an inner diameter of 2.7 mm;the dwelling time in the tube was about 25 minutes.

In the second reactor, a second metal alkoxide component, preferablytetraisopropyl orthotitanate, was mixed with an alcohol, preferablyethanol.

The product streams (A) and (B) were combined through another T pieceand continuously mixed by means of a static mixing tube. The presol wassubsequently degassed, and after addition of water, it was immediatelyfilled into molds of PP (2 x 2 x 2 cm), which were sealingly closed.After about 10 seconds, gelation started. After a dwelling time of ninedays in the closed molds, the lids were provided with holes, and the gelbodies were dried within the molds in the course of four days, while thetemperature was increased from room temperature to 120° C. The xerogelswere subsequently sintered to glass in a preheated sintering furnaceover the following temperature gradient: 100 to 800° C. (7 hours), 800°C. (0.5 hour), 800 to 1030° C. (4.6 hours), 1030° C. (1 hour).

A glass having a composition of SiO_(2/)TiO₂ = 80/20 (% by weight) wasobtained.

Example 2 SiO_(2/)ZrO₂ Glass

According to the process in Example 1, glasses of the followingcomposition were produced using zirconium(IV) butoxide: SiO_(2/)ZrO₂ =45/55 (% by weight) and SiO_(2/)ZrO₂ = 70/30 (% by weight). In thesecond reactor (PTFE flexible tube), zirconium(IV) butoxide was employedinstead of tetraisopropyl orthotitanate. Acetic acid (1 mol/l, V = 15.6%V_(presol)) was continuously added to the presol to start the gelation.After a dwelling time of four days in the closed molds, the lids of themolds were removed, and the molds were transferred to sealable dryingcontainers. For a controlled release of the solvents, the dryingcontainers had small openings (diameter 0.1 to 4.0 mm), while a liquid(water or a mixture of ethanol, 1-butanol, formamide and water) wasadded for atmospheric control. Subsequently, the gel bodies were driedwithin the molds in the course of four days, while the temperature wasincreased from room temperature to 120° C. The xerogels weresubsequently sintered to glass in a preheated sintering furnace over thefollowing temperature gradient: 100 to 800° C. (7 hours), 800° C. (0.5hour), 800 to 1030° C. (4.6 hours), 1030° C. (1 hour).

The FIGURE shows transmission data of inventive products in comparisonto conventional silica glass. The compositions of the products aresummarized in Table 1. The metal oxide component was ZrO₂. As can beseen, the inventive products have a transparency comparable to that ofconventional silica glass, despite comprising high amounts of metaloxide component. Further, it can be depicted from the data summarized inTable 1 that the refractive index nD of the inventive products could becontrolled by the addition of a metal oxide component.

Transmission was determined by UV-vis spectrometer with a thickness ofthe sample of 5 mm.

The refractive index was determined using a refractometer.

TABLE 1 product metal oxide component [wt.-%] density [g/cm³] nD 1 21.22.332 1.5226 2 27.3 2.405 1.5428 3 32.0 2.567 1.5616 4 37.2 2.717 1.58335 42.5 2.862 1.6066 6 52.6 3.093 1.6604

It is claimed:
 1. A continuous sol-gel method for the production ofsilicate-containing glasses and glass-ceramics comprising the steps of:(a) continuously feeding a mixture of tetraethyl orthosilicate andmethyltriethoxysilane and alcohol into a first reactor R1, and at leastpartially hydrolyzing by adding a mineral acid to obtain a first productstream A; the weight ratio of tetraethyl ortho-silicate to themethyltriethoxysilane is 5:1 to 15:1; (b) continuously providing asecond product stream B in a second reactor R2 by adding a metalalkoxide component or continuously mixing the alcohol and metal alkoxidecomponents; (c) continuous mixing of product streams A and B in a thirdreactor R3 to produce a pre-sol to obtain a third product stream C; (d)continuously adding water or dilute acid to product stream C to obtainsol gelation; (e) continuously filling the obtained sol into a mould toobtain a hydrogel; (f) drying the hydrogel to obtain a xerogel; (g)sintering the xerogel at 850 to 1200° C. to obtain a silicate-containingglass and glass-ceramic; wherein the metal oxide component is present inthe silicate-containing glass or silicate-containing glass-ceramic in anamount of 20 wt.% to 55 wt.%.
 2. The method of claim 1, wherein nitricacid is used as the mineral acid in the step (a).
 3. The methodaccording to claim 1, characterized in that in the step (a), 1 to 60% ofthe inorganic acid by weight of the silicon alkoxide is used.
 4. Themethod according to claim 1, wherein the at least partial hydrolyzing ofthe silicon alkoxide in the step (a) is carried out at a temperature of1-100° C.
 5. The method of claim 1, wherein in the step (b), an alkylorthotitanate is used as the metal alkoxide component.
 6. The methodaccording to claim 1, wherein in the step (b), zirconium (IV) alkoxideis used as the metal alkoxide component.
 7. The method of claim 1,wherein the product streams (A) and (B) are mixed in a volume ratio ofmetal alkoxide to silica of 10:1 to 1:10.
 8. The method of claim 1,wherein the product streams (A) and (B) are mixed in a temperature rangeof 0-80° C.
 9. The method of claim 1, wherein at least one of steps (a),(b) or (c) is carried out in a continuous reactor, which may have anupper agitator element.
 10. The method of claim 9, wherein thecontinuous reactor has a length of 1 cm to 1000 m and/or across-sectional width of 1 to 10 mm.
 11. The method of claim 1, whereinthe gelation is carried out in a temperature range of 0 to 100° C. 12.The method of claim 1, wherein the drying is carried out in atemperature range of 0-150° C.
 13. A silicate-containing glass or asilicate-containing glass-ceramic obtainable by a continuous sol-gelmethod comprising the steps of: (a) continuously feeding a mixture oftetraethyl orthosilicate and methyltriethoxysilane and alcohol into thefirst reactor R1, and at least partially hydrolyzing by adding a mineralacid to obtain a first product stream A; the ortho-silicon The weightratio of tetraethyl acid to the methyltriethoxysilane is 5:1 to 15:1;(b) continuously providing a second product stream B in the secondreactor R2 by adding the metal alkoxide component or continuously mixingthe alcohol and metal alkoxide components; (c) continuous mixing ofproduct streams A and B in a third reactor R3 to produce a pre-sol toobtain a third product stream C; (d) continuously adding water or diluteacid to product stream C to obtain sol gelation; (e) continuouslyfilling the obtained sol into a mould to obtain a hydrogel; (f) dryingthe hydrogel to obtain a xerogel; (g) sintering the xerogel at 850 to1200° C. to obtain a silicate-containing glass and glass-ceramic;wherein the metal oxide component is present in the silicate-containingglass or silicate-containing glass-ceramic in an amount of 20 wt.% to 55wt.%.
 14. The silicate-containing glass or silicate-containingglass-ceramic according to claim 13, wherein a refractive index nd ofthe silicate-containing glass or silicate-containing glass-ceramic is1.45-1.8.
 15. The silicate-containing glass or silicate-containingglass-ceramic according to claim 14, wherein the refractive index nd ofthe silicate-containing glass or silicate-containing glass-ceramic is1.48-1.75.
 16. The silicate-containing glass or silicate-containingglass-ceramic according to claim 15, wherein the refractive index nd ofthe silicate-containing glass or silicate-containing glass-ceramic is1.5 to 1.7.
 17. The silicate-containing glass or the silicate-containingglass-ceramic according to claim 13, wherein the silicate-containingglass or the silicate-containing glass-ceramic is resistant to lightwith a wavelength of 300 to 900 nm is transparent.
 18. Thesilicate-containing glass or the silicate-containing glass-ceramicaccording to claim 17, wherein the silicate-containing glass or thesilicate-containing glass-ceramic is resistant to light with awavelength of 350 to 850 nm is transparent.
 19. The silicate-containingglass or the silicate-containing glass-ceramic according to claim 18,wherein the silicate-containing glass or the silicate-containingglass-ceramic is resistant to light with a wavelength of 380 to 780 nmis transparent.
 20. The silicate-containing glass or silicate-containingglass-ceramic according to claim 13, wherein the silicate-containingglass or silicate-containing glass-ceramic has a light transmittance inthe visible light range, andthe difference in light transmittance withquartz glass in the visible light range is not more than 30%.